The present disclosure generally relates to methods and apparatus for determining the heart rate of a subject. More specifically, the present disclosure particularly relates to a method and apparatus for determining the beat-to-beat heart rate of a fetus.
Fetal monitoring (i.e., monitoring of the fetal condition during gestation and at birth) usually comprises monitoring uterine activity and the fetal beat-to-beat heart rate. The fetal heart rate, which provides an indication of whether the fetus is sufficiently supplied with oxygen, is preferably calculated from beat to beat.
To obtain a signal indicative of the fetal heart rate prior to rupture of the membranes, a noninvasive monitoring technique must be used. The most widely adopted measurement technique involves measuring the Doppler shift of an ultrasound signal reflected by the moving fetal heart.
In accordance with a known ultrasonic detection technique, an ultrasound transducer is placed externally on the pregnant woman's abdomen and oriented such that the transmitted ultrasound waves impinge upon the fetal heart. The reflected ultrasound waves are received either by the same or by a different ultrasound transducer. The Doppler shift of the reflected ultrasound wave is directly related to the speed of the moving parts of the heart, e.g., the heart valves and the heart walls.
Although the Doppler ultrasound is widely accepted and generally accepted method of monitoring fetal heart rate, ultrasound fetal heart rate monitoring has several drawbacks. One of these drawbacks is that the ultrasound fetal monitor transducer may not be able to monitor the fetal heart rate of a fetus in the case of an obese mother since the distance from the mother's skin surface to the fetal heart may be greater than the monitoring depth of the fetal heart rate monitor. Alternatively, ultrasonic fetal heart rate monitors that use a higher dose of ultrasound energy to increase the depth of sensing expose normal or underweight patients to a higher degree of ultrasonic energy than may be otherwise required.